03-13-2015, 10:15 AM
Eric, there are separate sections on the tuning cap for FM and AM, SW. The FM section is entirely independent. Both AM and SW use the same section, so it is unusual that SW works and AM doesn't.
You can try localizing the problem and see if its the oscillator or antenna sections causing the problem. Connect your scope probe to pin 4 of the 7F8 converter tube. There should be a 2.2k resistor there to ground. You should see the oscillation at that point. Rotate the tuning cap and see if the signal drops out on the scope as you tune below 700 kHz.
If it does, then the osc is the problem. If the signal remains below 700 kHz, but the noise and reception problem is evident then its most likely the ant section causing the problem
Good luck!
You can try localizing the problem and see if its the oscillator or antenna sections causing the problem. Connect your scope probe to pin 4 of the 7F8 converter tube. There should be a 2.2k resistor there to ground. You should see the oscillation at that point. Rotate the tuning cap and see if the signal drops out on the scope as you tune below 700 kHz.
If it does, then the osc is the problem. If the signal remains below 700 kHz, but the noise and reception problem is evident then its most likely the ant section causing the problem
Good luck!
